Doubly-fed variable speed constant frequency generator sets are one kind of wind generator sets that have the highest market share at present, and the pitch angles of the wind wheels thereof can be adjusted. Generators can operate at a variable speed and output electric energy with constant frequency and constant voltage, and the speed adjustment range thereof reaches ±30% of synchronous speed. A converter is an important component of a wind power generation device, and an AC-DC-AC converter with a back to back structure is generally used, wherein the part connected to a grid is called a line side converter (LSC), and the part connected to a rotor is called a rotor side converter (RSC). The magnitude and frequency of excitation field of rotors are changed by controlling the magnitude and frequency of exciting current of the rotors through the RSC, which make the output voltage and frequency of the generator keep pace with those of the grid. Thus, the wind power generation device operates at a variable speed and a constant frequency.
The power outputted by the doubly-fed power generation system to the grid consists of two parts, i.e., the power directly outputted from the stator and the power generated from the rotors through a converter. Converters only provide part of the power of the entire power generation system, and the ratio of the power outputted from the stator to the power generated through a converter varies as the rotate speed of the generator varies.
Among doubly-fed power generation systems whose capacity is less than 3MW, the voltage at stator side of a generator is 690 Vac, and the generator is connected with a low voltage transformer. The voltage at rotor side of the generator varies in a range between 0 and 690V according to the rotate speed of the generator. The rated AC voltage of the converter is 690V, and the DC bus voltage is generally 1100V.
Please refer to FIG. 1, which is a schematic view showing the structure of a doubly-fed wind power generation system in the prior art. As shown in FIG. 1, the current wind power generation and transmission system generally has a three-stage structure:
A doubly-fed wind power generation device 1a is as the first-stage structure, and includes doubly-fed generators 11a, converters 12a, fan blades 13a and gears. The doubly-fed wind power generation device 1a is mounted in a wind turbine tower to convert wind energy into electrical energy.
A first booster system 2a is as the second-stage structure, and boosts the low voltage alternating current (e.g. 0.69 kV) converted by the generators to medium voltage alternating current (e.g. 10 kV, 20 kV or 35 kV, etc.) through boosting transformers 21a. The capacity of the boosting transformer 21a is the same as that of the doubly-fed generator 11a. That is, the capacity of the first booster system 2a is the same as that of the doubly-fed wind power generation device la. The first booster system 2a is generally mounted in the vicinity of the tower.
A second booster system 3a is as the third-stage structure, and boosts the medium voltage alternating current to high voltage alternating current (e.g. 110 kV or 220 kV, etc.) through a transformer 31a. And the high voltage alternating current can be used for long-distance power transmission. The doubly-fed wind power generation device 1 a contains plurality of generators connected in parallel and each of the plurality generators connects with a boosting transformer 21a of the first booster system 2a, which form a wind power plant. And the wind power plant is connected to transmission and distribution lines via the common third-stage booster station.
The electric energy outputted by the doubly-fed generators 11a varies with the wind speed, and the converters 12a modulate the electric energy into alternating current with constant frequency and constant voltage. The boosting transformers 21a are mounted within a box transformer substation outside the tower. The boosting transformers 21a boost the low voltage alternating current of the doubly-fed wind power generation device and output the boosted alternating current at the medium voltage sides of the boosting transformers 21a. Then the medium voltage alternating current is converted into high voltage alternating current through the boosting transformer 31a, and the high voltage alternating current is inputted to the high voltage grid via the high voltage side of the boosting transformer 31a. The low voltage is 690V, the medium voltage is generally 10KV, 20KV or 35KV, and the high voltage is 110KV or 220KV. The point between the second booster system 3a and the first booster system 2a is called a medium voltage point. At the medium voltage point, generator sets can be connected in parallel to increase the generating capacity of the system.
In the existing doubly-fed wind power generation system, the alternating current outputted from a doubly-fed power generation device goes through two-stage booster systems, so the transmission efficiency is low and extra electric power will be lost. As the capacity of a single generator becomes higher and higher (e.g. a 3-6MW), the capacity and volume of the converter become larger accordingly. However, the diameter of the tower bottom, that is, the space where the converter is mounted, does not increase as the power multiplies. It is more and more difficult to design and mount the converter. With the increase in power, the current increases in proportion, and the number and capacity of the transmission cables needed increase in proportion, resulting increased construction costs of the existing doubly-fed wind power generation system.
Therefore, there is an urgent need to develop a wind power generation system which can overcome the above deficiencies.